1. Field of the Invention
The present invention relates to the treatment of retinal diseases, and more particularly, to the treatment of macular degeneration with prostaglandin derivatives.
2. Statement of the Problem
In the retina, the macula lutea, or macula retinae, is a small, irregular, yellowish area about three degrees wide, or less than 1 mm.sup.2. The macula lutea lies slightly lateral to the area in the center of the retina that constitutes the region of maximum visual acuity. This area is made up almost wholly of retinal cones and is important for color vision.
The major cause of blindness in people over the age of sixty in the United States is age-related macular degeneration. Macular degeneration results in a loss of central vision in both eyes in a typically slow process with continuous progressive loss of the vision. The visual loss is produced by pathological changes in the macula lutea. Macular degeneration is characterized by spots of pigmentation (druscen) or other abnormalities. The disease has a genetic factor.
The incidence of macular degeneration decreases in relationship to the darkness of eye color. Macular degeneration almost never afflicts African-Americans, and it occurs much less often in individuals with brown irises. The greatest frequency is found in persons with blue irises.
Macular degeneration is conveniently divided into three stages. It may present as early stage macular degeneration. At least 20% of patients develop an active neurovascular form.
One end stage of this condition is known as atrophic macular degeneration or "dry" macular degeneration, while a second end stage is known as neovascular macular degeneration or "wet" macular degeneration, which is caused when new blood vessels begin to grow under the retina, particularly the macula.
There are no effective treatments for dry macular degeneration.
Wet macular degeneration is treated by various methods, for example, laser treatment. However, a complication of laser treatment is actual loss of vision. In addition, laser treatment, where applicable, is not always a permanent cure since the blood vessels may begin to grow again.
Another type of treatment for wet macular degeneration involves injecting a photo-activated chemical into the general circulation of the patient and then exposing the patient to light, activating the chemical, which then kills the overgrowing blood vessels. This treatment has the unwanted side effect of making the patient extremely sensitive to light.
Anti-angiogenesis therapy has also been attempted as a treatment for wet macular degeneration. Because of the preliminary nature of these experiments, it is not yet known whether this treatment will be effective.
A surgical treatment of macular degeneration involves rotating the retina away from the area of blood vessel growth. Another surgical treatment involves performing a retinal cell transplant. These treatments subject the patient to surgical insult, and neither has been shown to be effective in preventing continuing degeneration of the macula.
A recent study has indicated that foods rich in carotenoids, especially lutein and zeaxanthin, the only pigments found in the macula lutea, may protect against the development of macular degeneration. Other treatments that have been attempted to prevent or treat macular degeneration include the use of vitamins A and E. None of these other treatments with anti-oxidants has been shown to be effective in the treatment or prevention of macular degeneration.
Thus, although age-related macular degeneration is the most prevalent cause of blindness in the elderly population in developed countries, there is no effective treatment that delays the course of or prevents the development of macular degeneration.
Prostaglandin derivatives have been used for some time to treat ocular hypertension (glaucoma). One of these derivatives, latanoprost, is a prostaglandin F.sub.2.alpha. derivative. The chemical name of latanoprost is isopropyl-(Z)-7[(1R,2R,3R,5S)3,5-dihydroxy-2-[(3R)-3-hydroxy-5-phenylpenty l]cyclo-pentyl]-5-heptenoate. The molecular formula is C.sub.26 H.sub.40 O.sub.5 and the molecular weight is 432.58. The chemical structure is illustrated in FIG. 1. The chemical structure of latanoprost is disclosed in PCT WO 90/02553, and a process for making latanoprost is disclosed in PCT WO 93/00329. A novel process for making latanoprost is taught in U.S. Pat. No. 5,466,833 to Ivanics et al., and the use of latanoprost in treating glaucoma is disclosed in U.S. Pat. No. 5,510,383 to Bishop et al.
It is also known that prostaglandin F.sub.2.alpha. derivatives have the ability to stimulate melanogenesis in tissues to which they are applied (see, for example, U.S. Pat. No. 5,905,091 to Fuller). For example, the application of latanoprost to the eye during the treatment of glaucoma often results in increased pigmentation of the eye, that is, in light-colored eyes with blue irises, the irises can become brown. This effect of prostaglandin F.sub.2.alpha. derivatives is discussed in the drug insert for the XALATAN.TM. latanoprost ophthalmic solution from Pharmacia & Upjohn This melanogenistic property has been seen as a negative side effect of the use of prostaglandin F.sub.2.alpha. derivatives, and it is suggested that XALATAN.TM. treatment be discontinued if increased pigmentation ensues during treatment (XALATAN.TM. drug insert). Solutions to overcome this problem are discussed in U.S. Pat. No. 5,886,035 to Shirasawa et al.
Solution to the Problem.
The present invention discloses a method of treating age-related macular degeneration with prostaglandin F.sub.2.alpha. derivatives, and preferably with latanoprost. This method is based on the property of prostaglandin F.sub.2.alpha. derivatives discussed above in which these derivatives cause the iris and other tissues to darken when applied topically to the eye.